The present invention relates to a multilayer insulated wire whose insulating layers are composed of two or more extrusion-coating layers. The present invention also relates to a transformer in which said multilayer insulated wire is utilized. More specifically, the present invention relates to a multilayer insulated wire that is useful as a lead wire and a winding of a transformer incorporated, for example, in electrical/electronic equipment; said wire has good solderability at a low temperature and a short period of time, at which an adverse effect is not easily produced on other members at the time of processing into a coil, and said wire is excellent in heat resistance, high-frequency characteristic, winding processing resistance, and solvent resistance. The present invention also relates to a transformer that utilizes said multilayer insulated wire.
The construction of a transformer is prescribed by IEC (International Electrotechnical Communication) Standards Pub. 60950, etc. That is, these standards provide that at least three insulating layers be formed between primary and secondary windings in a winding, in which an enamel film which covers a conductor of a winding be not authorized as an insulating layer, or that the thickness of an insulating layer be 0.4 mm or more. The standards also provide that the creeping distance between the primary and secondary windings, which varies depending on the applied voltage, be 5 mm or more, that the transformer withstand a voltage of 3,000 V applied between the primary and secondary sides for a minute or more, and the like.
According to such the standards, as a currently prevailing transformer has a structure such as the one illustrated in a cross-section of FIG. 2. In the structure, an enameled primary winding 4 is wound around a bobbin 2 on a ferrite core 1 in a manner such that insulating barriers 3 for securing the creeping distance are arranged individually on the opposite sides of the peripheral surface of the bobbin. An insulating tape 5 is wound for at least three turns on the primary winding 4, additional insulating barriers 3 for securing the creeping distance are arranged on the insulating tape, and an enameled secondary winding 6 is then wound around the insulating tape.
In recent years, however, a transformer having a structure that neither includes an insulating barrier 3 nor an insulating tape layer 5, as shown in FIG. 1, has begun to penetrate rapidly into the market, instead of the transformer having the sectional structure shown in FIG. 2. The transformer shown in FIG. 1 has an advantage over the one having the structure shown in FIG. 2 in being able to be reduced in overall size and dispense with the winding operation for the insulating tape.
In manufacturing the transformer shown in FIG. 1, it is necessary, in consideration of the aforesaid IEC standards, that at least three insulating layers 4b (6b), 4c (6c), and 4d (6d) are formed on the outer peripheral surface on one or both of conductors 4a (6a) of the primary winding 4 and the secondary winding 6 used.
As such a winding, a winding in which an insulating tape is first wound around a conductor to form a first insulating layer thereon, and is further wound to form second and third insulating layers in succession, so as to form three insulating layers that are separable from one another, is known. Further, a winding in which a conductor enameled with polyurethane is successively extrusion-coated with a fluororesin, whereby extrusion-coating layers composed of three layers structure in all are formed for use as insulating layers, is known (JU-A-3-56112 (xe2x80x9cJU-Axe2x80x9d means unexamined published Japanese utility model application)).
In the above-mentioned case of winding an insulating tape, however, because winding the tape is an unavoidable operation, the efficiency of production is extremely low, and thus the cost of the electrical wire is conspicuously increased.
In the above-mentioned case of extrusion of a fluororesin, since the insulating layer is made of the fluororesin, there is the advantage of good heat resistance and high-frequency characteristic. On the other hand, because of the high cost of the resin and the property that when it is pulled at a high shearing speed, the external appearance is deteriorated, it is difficult to increase the production speed, and like the insulating tape, the cost of the electric wire becomes high. Further, in this case of the insulating layer, there is a problem that, since the insulating layer cannot be removed by dipping in a solder bath, the insulating layer on the terminal has to be removed using less reliable mechanical means, and further the wire must be soldered or solderless-connected, when the terminal is worked for the insulated wire to be connected, for example, to a terminal.
On the other hand, a multilayer insulated wire is put to practical use, wherein multilayer extrusion-insulating layers are formed from a mixture of a polyethylene terephthalate as a base resin with an ionomer prepared by converting part of carboxyl groups of an ethylene/methacrylic acid copolymer to metal salts, and wherein the uppermost covering layer among the insulating layers is made of an aliphatic polyamide (nylon). This multilayer insulated wire is excellent in cost of electrical wire (nonexpensive materials and high producibility), solderability (to make possible direct connection between an insulated wire and a terminal), and coilability (that means that, in winding the insulated wire around a bobbin, the insulating layer is not broken to damage the electrical properties of the coil, when, for example, parts of the insulated wire are rubbed with each other or the insulated wire is rubbed with a guide nozzle) (JP-A-6-223634 (xe2x80x9cJP-Axe2x80x9d means unexamined published Japanese patent application)).
Recently, however, as a bobbin used in these transformers, a resin material having a low heat resistance has started to be used, from the viewpoint of recycling. When a conventional multilayer insulated wire is used in such a transformer, there may arise a problem that an adverse effect is produced on other members at the temperature and time necessary for processing into a coil. Thus, needs for a multilayer insulated wire having solderability at a low temperature and a short time, have been increasing.
The present invention is a multilayer insulated wire, which comprises a conductor and two or more solderable, extruded insulating layers with which the conductor is coated, wherein the first insulating layer nearest to the conductor is composed of a thermoplastic polyester elastomer resin and the outermost insulating layer is composed of a thermoplastic polyamide resin.
Further, the present invention is a transformer, wherein the above multilayer insulated wire is utilized.